Biomedical Engineering Reference
In-Depth Information
8.5.3 Detection limit
For the miniature PHSS, the minimum H
2
S detection limit in anoxic 10 mM PBS at
pH 7.0 and 37ºC at maximum amplifi er gain was near 10 nM H
2
S (Fig. 8.7 inset), and
the signal was linear over a concentration range of 0 to 1200 nM H
2
S (Fig. 8.7), and
additionally up to 200
M (not shown) [36]. If sample pH is increased, the proportion
of H
2
S species is decreased although the PHSS detection limit remains constant. At
100 nM H
2
S detection limit at pH 7.8, the PHSS compares favorably with standard
colorimetric methods [37].
µ
8.5.4 Stability
The spontaneous oxidation of H
2
S in aerated solutions is approximated with second
order reaction kinetics and can be detected as a slowly declining PHSS signal even in
low
M O
2
conditions in the presence of metal chelators such as DTPA. Therefore
to determine PHSS signal drift, a fl ow-through system was designed to limit spon-
taneous H
2
S oxidation. To do this, a syringe pump (model 22, Harvard Apparatus,
Holliston, MA) delivered a concentrated H
2
S stock from a Pyrex syringe at a constant
rate to a 3 ml constant volume fl ow-through chamber. The H
2
S stock solution was
7.5 mM Na
2
S in 500 mM Tris-buffered anoxic analytical grade water, pH 7.0. Delivery
rates were adjusted between 0.1 and 0.2 ml/h to create a steady-state H
2
S concentration
between 20 and 100
1
µ
µ
M, determined by 2-PDS [43]. PHSS signal drift was measured
at
5 pA/h for up to 10 hours, corresponding to
5 nM/h.
8.5.5 Reproducibility, precision and accuracy
To determine the precision of the PHSS response, 5 to 10 successive 2
L injections
of a 10 mM anoxic stock were made into a 2 mL anoxic chamber at pH 7.3 and 37ºC,
µ
150
8
6
4
100
2
0
0 0 0 080
Sulfide, nM
50
y
1.14
0.11x R
0.99
0
0
200
400
600 800
1000
1200
1400
Sulfide, nM
FIGURE 8.7
PHSS calibration. A representative PHSS calibration regression shows linearity of the signal
current from 0 to 1200 nM H
2
S. Inset: An expanded segment of the calibration trace shows the PHSS detection
limit near 10 nM H
2
S with a sensor that exhibited low background current at high amplifi er gain (after [41]).
Search WWH ::
Custom Search